The shader and 3D texture are for ray marching which is a method for rendering volumetric objects (objects described by voxels). That's not where the problem is, I already mocked up a version in RenderMonkey but I need to test solutions for objects that are within the volume, and for when the camera is within the volume.

As you can see in the shader I'm trying to do something as simple as get the value of the voxel in the middle. All of the voxels are coloured white so the fact that I get a black value means there's some problem binding or accessing the texture.